An electrical-hydraulical servo-valve (hereinafter referred to as "a hydraulic servo-valve") has been widely heretofore used, e.g., for numerical control of a machine tool or remote control, by converting a weak intensity electrical input signal into hydraulic pressure. With the converted hydraulic pressure, the hydraulic servo-valve changes the direction of flow of a working liquid and moreover changes a flow rate of the working liquid. A few examples of the conventional hydraulic servo-valves will be described below with reference to FIGS. 1 to 3.
Referring to FIGS. 1 and 2, the hydraulic servo-valve is fed with pressurized hydraulic oil via a pump port P. When e.g. a coil 22R of a torque motor 21 is magnetized in response to an electrical input signal, a movable shaft 24 is displaced in the rightward direction, whereby the lower end 20Ra of a flapper 20R is displaced in the leftward direction. Thus, pressure in a nozzle back-pressure chamber 18R is increased and moreover pressure in a pilot chamber 13R is also increased. As a result, a spool 10 is displaced in the leftward direction so that the hydraulic oil is introduced in the interior of a hydraulic cylinder (not shown) from the pump port P via a cylinder port C1. On the other hand, the hydraulic oil returning from the hydraulic cylinder returns to a tank (not shown) from a cylinder port C2 via a passage 5 and a tank port R. In addition, the hydraulic oil flowing from the gap between the nozzle 19R and the flapper 20R returns to the tank from the tank port R via a passage 6.
FIG. 3 is a view which schematically illustrates another conventional hydraulic servo-valve. This hydraulic servo-valve is provided with an opposing pair of nozzles on both sides of a flapper. Referring to FIG. 3, as the hydraulic servo-valve is fed with hydraulic oil via a pump port P, the hydraulic oil flows through passages 26L and 26R in a valve body 1 so that it is introduced into nozzle back-pressure chambers 18L and 18R via orifices 27L and 27R for controlling back-pressure. The hydraulic oil discharged from the gaps between nozzles 19L and 19R and a flapper 20 returns to a tank (not shown) via passages 6L and 6R and tank ports R1 and R2. When the flapper 20 is displaced, e.g., in the leftward direction in response to an electrical signal inputted into a torque motor 21, pressure in the nozzle back-pressure chamber 18L is increased and moreover pressure in a pilot chamber 13L is also increased. On the other hand, pressure in a nozzle back-pressure chamber 18R is reduced and moreover pressure in a pilot chamber 13R is reduced. Thus, a spool 10 slidably received in a sleeve 2 is displaced in a rightward direction against the resilient force of a spring 28R. As a result, the hydraulic oil is introduced into the interior of a cylinder (not shown) from the pump port P via a cylinder port C1. On the other hand, the hydraulic oil returning from the hydraulic cylinder returns to a tank (not shown) from a cylinder port C2 via tank port R2.
Since the hydraulic oil serving as a working liquid is very inflammable, care must be taken during handling of the hydraulic oil. Waste hydraulic oil may cause environmental contamination.
In the past, water was used as a working liquid for driving or controlling a hydraulic machine. However, in a case where water serves as a working liquid, since water has a low viscosity, there arise problems that a large quantity of water leaks through a clearance S between a spool and a sleeve, resulting in a low rate of efficiency, slidable portions are subject to wear due to friction and a hydraulic machine fabricated using a metallic material (particularly, ferrous material) is liable to rust, if it is left unused.
In recent years, considerable advances in the production of new raw materials have been made, e.g., plastics. Accordingly, one of the aforementioned problems, i.e., rust, appearing in the case where water is employed as a working liquid can be satisfactorily solved by fabricating portions coming into contact with a working liquid in a hydraulic machine from a new raw material. However, the problem concerning wear due to the low visocity of the working liquid (water) is still left unsolved. In addition, it is difficult to machine slidable portions with a high decree of accuracy for the purpose of minimizing leakage of the working liquid.
With the conventional hydraulic servo-valve as shown in FIGS. 1 and 2, the stroke of the spool 10 cannot be made large, because the hydraulic servo-valve has a narrow gap between the nozzle 19R and the flapper 20R. For this reason, the hydraulic servo-valve cannot be designed to have a high flow rate. Another problem is that the flapper mechanism has low responsiveness due to a large amount of working liquid leakage through the clearance S, S1, between the spool and the sleeve.
The present invention has been made with the foregoing problems in mind and its object resides in providing a hydraulic servo-valve wherein water can be used as a working liquid, problems concerning wear, rusting and leakage have satisfactorily been solved and responsiveness of the flapper mechanism has been improved.